I am in the process of rebuilding all of the charging system in one of mine and putting a "battery bank" under the cab....Needless to say I am building all of my own cables etc. Long story short I when I couldnt find what I wanted in the standard automotive world I turned to the car audio scene. You gotta do some shopping cuz there is massive markup, but if you can find a decent wholesaler they will have pretty much anything you can dream of dirt cheap.
I am building a 4 battery bank, and spent $120 on terminals for 4 standard 2/0's, 3 dual line 2/0's with a top lug, and 1 dual 2/0 & dual 4ga....will probably spend another $100 on the cable